1. Field of the Invention
The present invention relates to a variable valve mechanism which allows switching of cams driving a valve by selection of rocker arms.
2. Description of the Related Art
In a reciprocating internal combustion engine, a cam switching type variable valve mechanism is used in a valve train in order to achieve both improvement in output and improvement in fuel consumption. The cam switching type variable valve mechanism is an apparatus which switches opening/closing timings of a valve (a intake valve or an exhaust valve) or valve lifts by switching of cams. In a representative variable valve mechanism, a plurality of cams different in cam profile are provided on one camshaft. Opening/closing timings of a valve or valve lifts are switched by switching rocker arms for each cam driven by these cams.
In an engine, a camshaft may be arranged on an obliquely upper side of a rocker shaft in order to avoid interference among respective sections of the engine to achieve compactness of a cylinder head. In such an engine, a pin type variable valve mechanism is used so as not to injure compactness of the cylinder head.
One example of a conventional pin type variable valve mechanism is described in Jpn. Pat. Appln. KOKAI Publication No. 2000-345872. In the variable valve mechanism, one of a plurality of rocker arms adjacent to one another stores therein a pin which can project from/retract to the other rocker arm. The other rocker arm is formed with a recess in which a distal end of the pin projected is fitted. With this configuration, a cam displacement transmitted from the one rocker arm is transmitted to a valve at a time of retraction of the pin. When the pin is projected, the pin is fitted into the recess of the other rocker arm so that the both the rocker arms are coupled to each other. Thereby, a cam displacement transmitted from the other rocker arm is transmitted to the valve via the pin and the one rocker arm.
The configuration where two rocker arms adjacent to each other are coupled using the pin is excellent in compactness but there is a drawback in cost because of the following reasons.
In the pin type variable valve mechanism, in a coupled state of two rocker arms, the rocker arms are rotationally displaced to drive the valve while a shearing stress is being applied to the pin. For this reason, a relatively large shearing stress acts on the pin or the rocker arms. Therefore, in the pin type variable valve mechanism, means for securing a stable cam switching operation, such as remarkable improvement in mechanical strength of the pin or the rocker arms is required. Therefore, the pin type variable valve mechanism will cause increase in cost easily.